It is well known that star polymers containing polyisobutylene arms of practically any length, and thus, with controlled molecular weight, can be synthesized by living cationic polymerization. Such star polymers are gradually gaining acceptance and may have potential application in the motor oil industry as motor oil additives and viscosity index improvers, to the extent that they are known to decrease the viscosity of lubricating oils over a wide temperature range.
A number of patents have been obtained relating to star polymers, generally on the basis of providing a different, chemically crosslinked core from which the polyisobutylene arms emanate. For example, one of the first synthesis of a star polymer containing carbocationically polymerized polyisobutylene arms emanating from a crosslinked core is described in Kennedy et al. U.S. Pat. No. 10 5,395,885. There, excess divinylbenzene was used as a linking reagent to a living polyisobutylene charge by the "arm first" method. The resultant star polymer had a plurality of polyisobutylene arms emanating from a polydivinylbenzene (PDVB) core. The core, however, was ill-defined to the extent that its structure was not readily characterizable or controllable, due mainly to the presence of some double bonds in the core itself.
Consequently, other star polymers were synthesized having well-defined, chemically crosslinked cores which did not have any such undesirable double bonds. For example, Kennedy et al. U.S. Pat. No. 5,663,245 describes a star polymer having a plurality of polyisobutylene arms emanating from a siloxane core, while Majoros et al. U.S. Pat. No. 5,840,814 discloses a star polymer having a plurality of polyisobutylene arms emanating from a calixarene core.
Although the star polymers discussed hereinabove appear to demonstrate a high degree of resistance to decomposition, these star polymers, nevertheless, ultimately will decompose upon exposure to high heat or high stress external environments. This is because all of the previously synthesized star polymers have chemically crosslinked cores which were incapable of molecular shape change upon exposure to heat or other external influences. That is, all of the cores of the previous star polymers are "static" rather than "dynamic". Since all of the described star polymers have crosslinked, i.e., chemically bonded, cores, they necessarily lack an advantageous viscosity property desirable in lubricant additives, i.e., the ability to change molecular structure upon exposure to heat or other external influences, and then return to their original state upon the return of previous conditions.
In order to effectively change molecular structure, it will be appreciated that the "core" of the star polymer must have segments which can separate upon changes in external conditions. Just as important, the process must be reversible, meaning the core segments can relink upon a return of prior conditions.
The synthesis of block copolymers by living carbocationic polymerization of monomers such as isobutylene and an aromatic, preferably styrenic, monomer is also known in the art and has been described in at least Faust et al. U.S. Pat. No. 5,428,111. A more detailed discussion relating to the living carbocationic polymerization of polyisobutylene and block copolymers, particularly diblocks, can be found generally in Kennedy, J. P. and B. Ivan, Designed Polymers by Carbocationic Macromolecular Engineering: Theory and Practice, Hanser Publishers, New York, N.Y., (1992), the disclosure of which is incorporated herein by reference. Such block polymers and copolymers containing polyisobutylene have also been used as the "arms" of a number of star block polymers including those star block polymers having polyisobutylene-b-styrene block polymer arms emanating from a calixarene core as described in Marjoros et al. U.S. Pat. No. 5,840,814. However, in each instance, a crosslinked core, i.e., a chemically bonded core, has been a necessary part of the star polymer composition. Such a chemically crosslinked core is not suitably dynamic in character so as to change molecular shape upon exposure to external influences such as heat.
As such, there exists a need for a star polymer with a chemically uncrosslinked core which will not decompose, but rather will undergo molecular shape change at high temperatures, and a method for the synthesis of such a star polymer.